Chronic hepatitis C virus (HCV) infection is a major global health burden, with an estimated 170 million people infected worldwide and an additional 3 to 4 million infected each year (See e.g. World Health Organization Fact Sheet No. 164. October 2000). Although 25% of new infections are symptomatic, 60-80% of patients will develop chronic liver disease, of whom an estimated 20% will progress to cirrhosis with a 1-4% annual risk of developing hepatocellular carcinoma (See e.g. World Health Organization Guide on Hepatitis C. 2002; Pawlotsky, J-M. (2006) Therapy of Hepatitis C: From Empiricism to Eradication. Hepatology 43:S207-S220). Overall, HCV is responsible for 50-76% of all liver cancer cases and two thirds of all liver transplants in the developed world (See e.g. World Health Organization Guide on Viral Cancers. 2006). And ultimately, 5-7% of infected patients will die from the consequences of HCV infection (See e.g. World Health Organization Guide on Hepatitis C. 2002).
The current standard therapy for HCV infection is pegylated interferon alpha (IFN-α) in combination with ribavirin. However, only up to 50% of patients with genotype 1 virus can be successfully treated with this interferon-based therapy. Moreover, both interferon and ribavirin can induce significant adverse effects, ranging from flu-like symptoms (fever and fatigue), hematologic complications (leukopenia, thrombocytopenia), neuropsychiatric issues (depression, insomnia, irritability), weight loss, and autoimmune dysfunctions (hypothyroidism, diabetes) from treatment with interferon to significant hemolytic anemia from treatment with ribavirin. Therefore, more effective and better tolerated drugs are still greatly needed.
HCV, first identified in 1989 (See e.g. Choo, Q. L. et al. Science (1989) 244:359-362), is a single-stranded RNA virus with a 9.6-kilobase genome of positive polarity. It encodes a single polyprotein that is cleaved upon translation by cellular and viral proteases into at least ten individual proteins: C, E1, E2, p7, NS2, NS3, NS4A, NS4B, NS5A, and NS5B (See e.g. Lindenbach, B. D. et al. (2001). Flaviviridae: the viruses and their replication, p. 991-1041. In D. M. Knipe, P. M. Howley, and D. E. Griffin (ed.), Fields virology, 4th ed, vol. 1. Lippincott Williams & Wilkins, Philadelphia, Pa.).
NS3, an approximately 70 kDa protein, has two distinct domains: a N-terminal serine protease domain of 180 amino acids (AA) and a C-terminal helicase/NTPase domain (AA 181 to 631). The NS3 protease is considered a member of the chymotrypsin family because of similarities in protein sequence, overall three-dimensional structure and mechanism of catalysis. The HCV NS3 serine protease is responsible for proteolytic cleavage of the polyprotein at the NS3/NS4A, NS4A/NS4B, NS4B/NS5A and NS5A/NS5B junctions (See e.g. Bartenschlager, R., L. et al. (1993) J. Virol. 67:3835-3844; Grakoui, A. et al. (1993) J. Virol. 67:2832-2843; Tomei, L. et al. (1993) J. Virol. 67:4017-4026). NS4A, an approximately 6 kDa protein of 54 AA, is a co-factor for the serine protease activity of NS3 (See e.g. Failla, C. et al. (1994) J. Virol. 68:3753-3760; Tanji, Y. et al. (1995) J. Virol. 69:1575-1581). Autocleavage of the NS3/NS4A junction by the NS3/NS4A serine protease occurs intramolecularly (i.e., cis) while the other cleavage sites are processed intermolecularly (i.e., trans). It has been demonstrated that HCV NS3 protease is essential for viral replication and thus represents an attractive target for antiviral chemotherapy.